chg: nil: Improve reuse of outgoing TCP connections

This MR is a prerequisite for !8348.

It intentionally does not have a changelog entry associated with it, to
prevent making a false impression of improving connection reuse **for
existing code**.  It will only make a difference once !8348 gets merged
(and even then, only if the new `ISC_SOCKET_DETAILS` macro will be set
during build).  That's because `isc_nmhandle_localaddr()` currently
simply returns `handle->local` and its return value will only be set to
the actual address the socket is bound to with !8348 in place.

Note that `dns_dispatch_gettcp()` is currently only used by the
`dns_request` API, so this MR's potential for introducing new breakage
is relatively low.

Closes #4693

Merge branch '4693-improve-reuse-of-outgoing-tcp-connections' into 'main'

See merge request isc-projects/bind9!8972